This invention pertains to color laser imaging methods and apparatus and, in particular, to methods and apparatus for reducing or eliminating the interruption of normal printing capability during a calibration cycle.
Color printing by an electrophotographic printer is achieved by first scanning a digitized image onto a photoconductor. Typically, the scanning is performed with diodes which pulse a beam of energy onto the photoconductor. The diodes can be, for example, laser diodes or light emitting diodes (LEDs). The photoconductor typically comprises a movable surface coated with a photoconductive material capable of retaining localized electrical charges. In many cases, the movable surface is in the form of a revolvable cylindrical drum.
The surface of the photoconductor is divided into small units called pixels. The photoconductor is generally configured to continuously revolve such that any given pixel is repeatedly moved past the diodes at a substantially regular cycle and at a substantially constant rate, and along a substantially fixed path relative to the diodes. Each pixel is capable of being charged to a given electrical potential, independent of the electrical charge of each surrounding pixel.
During operation of the printer, substantially all of the pixels are first charged to a base electrical charge as they move past a charging unit during each revolution of the photoconductor. Then, as the pixels move past the diodes, the beam of energy, such as a laser, is either directed at, or not directed at, each of the pixels as dictated by the digital data the laser is directed at a given pixel, the given pixel can be electrically altered by changing (typically discharging) the base electrical charge to a second electrical charge.
Thus, after passing a laser during operation of the printer, a first portion of the pixels will remain at the base electrical charge because they were not exposed to the laser, while a second portion will have a different charge because of being altered by the laser. The first and second portions of unaltered and altered pixels thus form an image on the photoconductor. One portion of pixels will attract toner, while the other portion will not, depending on various factors such as the electrical potential of the toner. That is, the unaltered pixels will either attract or not attract toner, and vice versa with regard to the altered pixels.
In most electrophotographic printing processes, the altered, or electrically discharged, pixels attract toner onto the photoconductor. In this manner, toner is selectively transferred to the image on the photoconductor formed by the electrically discharged pixels. This process is known as discharge area development (DAD). However, in some electrophotographic printing processes toner is attracted to the un-discharged (i.e., charged) pixels on the photoconductor. This latter type of electrophotographic printing is known as charge-area-development (CAD). For purposes of discussion, it will be assumed that DAD is used, although the present invention is not limited to DAD.
Once the toner has been applied to the photoconductor, the toner is then transferred to a finished product medium, such as a sheet of paper. Although the finished product medium typically comprises paper, it can also comprise other materials such as plastic, as in the case of a transparency. The transfer of toner from the photoconductor to the finished product medium can be direct, or it can be indirect using an intermediate transfer device. That is, in the direct method, the toner is transferred directly from the photoconductor to the finished product medium. In the indirect method, the toner is transferred first to an intermediate transfer device, then transferred from the intermediate transfer device to the finished product medium. The intermediate transfer device typically comprises a revolvable endless belt. During operation of the printer, the intermediate transfer device typically moves by circulating, or revolving, past the photoconductor.
After the toner is transferred to the finished product medium, it is processed to fix the toner thereto. This last step is normally accomplished by thermally heating the toner to fuse it to the finished product medium, or applying pressure to the toner on the finished product medium. Any residual toner on the photoconductor and/or the intermediate transfer device is then removed by a cleaning station, which can comprise either or both mechanical and electrical means for removing the residual toner.
A variety of methods are known for selectively attracting toner to a photoconductor. Generally, each toner has a known electrical potential affinity. As described above, selected pixels of the photoconductor can be exposed by a laser from a base potential to a given potential associated with the selected toner, and then the toner can be presented to the photoconductor so that the toner is attracted only to the selectively exposed pixels. This latter step is known as developing the photoconductor.
In some processes, after the photoconductor is developed by a first toner, the photoconductor is then recharged to the base potential and subsequently exposed and developed by a second toner. In other processes, the photoconductor is not recharged to the base potential after being exposed and developed by a selected toner. In yet another process, the photoconductor is exposed and developed by a plurality of toners, then recharged, and then exposed and developed by another toner. In certain processes, individual photoconductors are individually developed with a dedicated color, and then the toner is transferred from the various photoconductors to a transfer medium which then transfers the toner to the finished product medium. The selection of the charge-expose-develop process depends on a number of variables, such as the type of toner used and the ultimate quality of the image desired.
Image data for an electrophotographic printer (which will also be known herein as a xe2x80x9cprinterxe2x80x9d), including color laser printers, is digital data which is stored in computer memory. The data is stored in a matrix or xe2x80x9crasterxe2x80x9d which identifies the location and color of each pixel which comprises an overall image. The raster image data can be obtained either by scanning an original analog document and digitizing the image into raster data, or by reading an already digitized image file. The former method is more common to photocopiers, while the latter method is more common to printing computer files using a printer. Accordingly, the invention described below is applicable to either photocopiers or printers.
Recent technology has removed the distinction between photocopiers and printers such that a single printing apparatus can be used either as a copier, a printer for computer files, or a facsimile machine. In any event, the image to be printed onto finished product media is provided to the printer as digital image data. The digital image data is then used to pulse the beam of a laser in the manner described above so that the image can be reproduced by the electrophotographic printing apparatus. Accordingly, the expression xe2x80x9cprinterxe2x80x9d should not be considered as limiting to a device for printing a file from a computer, but should also include any device capable of printing a digitized image in the general manner described herein, regardless of the source of the image.
The image data file is essentially organized into a two dimensional matrix within the raster. The image is digitized into a number of lines. Each line comprises a number of discrete points. Each of the points corresponds to a pixel on the photoconductor. Each point is assigned a binary value relating information pertaining to its color and potentially other attributes, such as density. The matrix of points makes up the resultant digitally stored image. The digital image is stored in computer readable memory as a raster image. That is, the image is cataloged by line, and each line is cataloged by each point in the line. A computer processor reads the raster image data in line-by-line fashion, and actuates the laser to selectively expose a given pixel based on the presence or absence of coloration, and the degree of coloration for the pixel.
The method of transferring the digital raster data to the photoconductor via a laser, lasers, or LEDs, is known as the image scanning process, or the scanning process. The scanning process is performed by a scanning portion, or scanning section, of the electrophotographic printer. The process of attracting toner to the photoconductor is known as the developing process. The developing process is accomplished by the developer section of the printer. Image quality is dependent on both of these processes. Image quality is thus dependent on both the scanning section of the printer, which transfers the raster data image to the photoconductor, as well as the developer section of the printer, which manages the transfer of the toner to the photoconductor.
In the case of a typical four-color laser printer, at least one laser scanner is included in the printer and utilized to generate a latent electrostatic image on the photoconductor. Generally, one latent electrostatic image is generated for each color plane to be printed. A xe2x80x9ccolor planexe2x80x9d generally refers to a portion of the output image which comprises only a single color of toner. In a four-color laser printer, the final output image comprises four color planes. This allows for each of four colors to be imaged first onto a photoconductor, then transferred onto an intermediate transfer device, and finally transferred from the intermediate transfer device to the finished product medium. As discussed above, in accordance with certain printer configurations, the intermediate transfer device is omitted and image is transferred directly from the photoconductor to the finished product medium.
Generally, two types of four-color laser printers exist. One type is a four-pass printer which has only a single photoconductor. Another type is an in-line printer which has four photoconductors. These two types will be discussed in further detail below. Both types of printers are generally configured to print images having four color planes. The four color planes typically printed, and which are generally considered as necessary to generate a relatively complete palate of colors, are yellow, magenta, cyan and black. That is, the typical color printer is provided with toners in each of these four colors. However, it is understood that some printer configurations employ fewer than four color planes while other printer configurations employ more than four color planes. Some printers have the capability of printing one color on top of another on the same pixel, so as to generate a fuller palate of finished colors.
In a typical scanning process, a laser is scanned from one edge of the photoconductor to the opposite edge while being selectively pulsed in accordance with the image data file. That is, the laser scans transversely across the photoconductor, following a row of pixels. As the laser scans along the row of pixels, it is selectively pulsed a pixel-by-pixel basis. That is, for each pixel in a row, the laser is either directed at the pixel, or not directed at it. The scan of the laser in this manner causes a line of point which make up the digital image to be transferred from the raster onto the photoconductor. As the photoconductor moves past the laser, the laser advances to the next row of pixels, and the next line of points from the digital image is scanned by the laser onto the photoconductor. The image data is thus scanned onto the photoconductor in a pixel-by-pixel and line-by-line basis until the complete image is transferred to the photoconductor.
The side-to-side scanning action of each laser is traditionally accomplished using a dedicated multi-faceted rotating polygonal mirror at which a stationary laser is aimed. The rotation or the mirror causes the reflected laser beam to be scanned across the photoconductor. at a unique relative lineal position from a first edge to a second edge of the photoconductor. As the mirror rotates to an edge of the polygon between facets, the reflected laser reaches the edge of the photoconductor. When the laser is reflected off of the next facet as it rotates into position, the laser is essentially reset to the first edge of the photoconductor to begin scanning a new line onto the advancing photoconductor.
In a color laser printer apparatus, there are several parameters that must be maintained in order to consistently produce color images of acceptable quality. One of these parameters is the registration, or alignment, of the different color planes. That is, each laser and photoconductor should be aligned with respect to the other lasers and photoconductors such that a given point in the raster image is associated with a single common point on the surface of the finished product medium. A printer having a color plane registration which is xe2x80x9coffxe2x80x9d will produce a blurry image, or an image with colors not representative of the original image.
Each laser and its associated components (i.e., rotating mirror, optical elements, and deflector mirror) is typically mounted in a precision housing to keep the components in relative fixed position with respect to one another. Assuring registration of the lasers requires aligning the four housings within the printer itself. As environmental conditions within the printer change (e.g., temperature), this alignment can change. Mechanical vibration or shock to the printer can also allows the lasers to become misaligned.
Since only partial alignment of the laser beams with respect to one another can be achieve by aligning the housings which contain the scanning assemblies, typical color printers are provided with an integral, on-board calibration system to allow for factory and ex-factory alignment of the lasers. One component of the calibration system is a plurality of color plane sensors to sense color plane registration. The sensors are provided to detect shifts in color planes in both the side-to-side scanning direction (the xe2x80x9cscanxe2x80x9d direction), as well as in the direction of advance of the photoconductor surface (i.e., the xe2x80x9cprocessxe2x80x9d direction). The sensors can provide a feedback to the scanning system and corrections, or adjustments, can be made to reposition the laser beams using various known electrical and mechanical methods.
In addition to color plane registration, color density is another parameter which must be maintain in order to produce accurate images. By xe2x80x9ccolor densityxe2x80x9d I mean the propensity of an area of applied toner to reflect light energy as a function of the amount of toner applied per unit area. Relatively high color density is generally associated with colors that can be described as dark, while relatively low color density is generally associated with colors that can be described as light. In order to faithfully reproduce an original image, the color density of the toners, as applied to the photoconductor, should be such that the brightness and contrast of the colors appear in the reproduced image as they are intended. Another related parameter that can be important to achieving a high quality reproduced image is faithful reproduction of the spectrum of the colors which are in the original image. That is, a color characterized by a given wavelength in the original image should preferably have essentially the same wavelength in the reproduced image.
Many factors, including atmospheric conditions, and variations in the toners themselves, can affect the spectral aspects of the finished product. This phenomenon is sometimes referred to as printer xe2x80x9cdrift.xe2x80x9d Thus, it is desirable to provide a mechanism to compensate for toner variations caused by printer drift. Such a mechanism can attempt to correct spectral variances by varying the mix of toners applied to a pixel, as well as the quantity of each toner applied. To determine when a color density or spectrum is accurately imaged, the calibration system of a printer can be further provided with color density sensors and color spectrum sensors which can detect the characteristics of a color (e.g., brightness, contract, gamma, and spectral characteristics).
In order to assist in determining whether the printer is reproducing the original image within acceptable limits, the calibration system is provided with a reference calibration image. The reference calibration image can comprise various patches of toner (calibration patches), each having associated characteristics of known specifications. These known specifications of the reference calibration image can be employed as base references against which the characteristics of reproduced images can be compared. For example, the reference calibration image can comprise various calibration patches, each having known color wavelengths and color densities against which actual reproduced calibration patches can be compared.
To use the reference calibration image, the calibration system can cause the printer to initiate a calibration cycle. Typically a calibration cycle is initiated by one of several possible events. For example, a calibration cycle can be initiated by turning the printer on or by the replacement of a toner cartridge. The calibration cycle can also be initiated by a timer or page counter, or the like. That is, the calibration cycle can be initiated by the passage of a preset interval of time or can be initiated when the number of printed pages reaches a specified number.
During the calibration cycle, the printer suspends the normal print mode and attempts to exactly reproduce the reference calibration image which can comprise a plurality of various calibration patches. After the calibration image is reproduced by the printer, a sensor or sensors measure the various characteristics of the reproduced calibration image, such as color density. The measured characteristics of the reproduced calibration image are then compared to the known characteristics of the reference calibration image.
If a discrepancy is detected between the reproduced calibration image and the reference calibration image, and if the discrepancy is outside of acceptable limits, the calibration system can attempt to adjust various parameters of the printer in an effort to minimize the discrepancies. For example, if a color density is determined to be inaccurate in the reproduced calibration image, the printer can adjust the application of the affected toner. Thus, the calibration image can also be described as a xe2x80x9ctest imagexe2x80x9d which is used to test whether the printer is producing images that are within acceptable specifications.
The reference calibration image is typically stored in computer readable memory which is preferably resident within the printer itself. When the calibration cycle is initiated, either automatically or as directed by a user, the printer retrieves the reference calibration image from the memory and then reproduces it. Generally, the calibration image is reproduced on one of the surfaces which normally bear output images during normal production. These surfaces can include photoconductors and intermediate transfer devices as well as the finished product medium.
For example, the calibration image can be reproduced on an intermediate transfer device, if the printer is so equipped. Once the calibration image is reproduced, it can then be moved past the calibration sensor(s) to detect and measure the various characteristics of the patch. Generally, the printer is provided with a cleaning station which is configured to remove the calibration patches from surfaces such as the intermediate transfer device after the characteristics of the patches are measured by the sensor(s).
Once the characteristics of the patches are measured, the calibration sensor(s) transmit the measured characteristics of the reproduced calibration image, in the form of output signals, to a processing unit (preferably resident within the printer). The output signals from the calibration sensor(s) can be stored temporarily in computer resident memory. The output signals are then compared to the reference calibration image to determine if the reproduced calibration image varies from the reference calibration image, and if so, by how much.
The processing unit can be further provided with an calibration algorithm to cause the calibration image to be produced and to determine what correction(s), if any, to the printer are required in order to bring the reproduced imaged within acceptable limits of accuracy. After any adjustments are made, the printer can be caused to reproduce the reference calibration image a second time in order to determine whether the corrective adjustments have brought the various components of the imaging apparatus into conformance so as to produce an image within the specifications of the reference calibration image. However, in an effort to minimize the time and cost associated with the calibration interval, calibration cycles commonly reproduce the calibration image only once during each calibration cycle.
As mentioned above at least two types of four-color laser printers are known. Two common types are the four-pass type and the in-line type. The four-pass type is generally provided with a single photoconductor and a single laser/mirror scanner system. The four-pass is also generally provided with a movable intermediate transfer device, commonly in the form of an endless belt which circulates, or revolves, past the photoconductor.
In operation, each of the four color planes (typically black, yellow, cyan, and magenta) which make up an output image is consecutively developed on the photoconductor and completely deposited on the intermediate transfer device. That is, as a first color plane is developed on the photoconductor, it is deposited in its entirety on the intermediate transfer device as the device makes a complete first revolution, past the photoconductor.
The intermediate transfer device then begins a second revolution past the photoconductor during which the second color plane is developed on the photoconductor and deposited in its entirety on the intermediate transfer device in registered alignment with the first color plane. This process is repeated in like manner for the third and fourth color planes until all four color planes have been deposited on the intermediate transfer device so as to build-up the completed image thereon. It is important that each succeeding color plate is deposited exactly xe2x80x9con top ofxe2x80x9d the previous color plate. That is, each succeeding color plate is superimposed, or deposited in registration with, the previous color plate. device, it is then transferred to a sheet of finished product medium. A characteristic of the four-pass printer is that the size of output image produced thereby is limited by the length of the intermediate transfer device, since the entire output image is produced in its entirety on the intermediate transfer device before the image is transferred to the finished product medium.
By comparison, a typical four-color, in-line type of printer is provided with four lasers and four in-line photoconductors. Each of the lasers is paired with one of the photoconductors. Also, each of the four colors of toner (typically black, yellow, cyan, and magenta) corresponds exclusively to one of the laser/photoconductor pairs. Like the four-pass type of printer, the in-line type generally has only one intermediate transfer device, which is also commonly in the form of an endless belt. However, unlike the multi-pass type of printer, the in-line type of printer does not require an intermediate transfer device for operation. Thus, in-line printers without intermediate transfer devices can be configured to transfer toner, in the form of an image, directly from the photoconductors to the finished product medium.
During operation of a typical in-line printer, each of the four color planes is developed on its own corresponding photoconductor and then deposited on the intermediate transfer device or, as in an alternative configuration, directly on the finished product medium. Generally, all of the color planes of a given image produced by an in-line printer are produced concurrently, as opposed to one-at-a-time as in a four-pass printer. By xe2x80x9cconcurrently,xe2x80x9d I mean the occurrence of a group of two or more events, each event having a duration over a time interval, and wherein at least a portion of the time interval of each event overlaps that of every other event in the group, and further wherein each event does not necessarily occur simultaneously.
Also, whereas the four-pass printer employs a step-by-step process to xe2x80x9cbuild upxe2x80x9d each output image one color plane at a time, the in-line printer employs a continuous process to produce the output image. That is, in an in-line printer having an intermediate transfer device, all four color planes of a given portion of the output image are deposited on the intermediate transfer device and transferred to the finished product medium in less than one revolution of the intermediate transfer device.
Unlike the case of four-pass printer configuration, the size of the image produced by the in-line printer is not limited by the size of the intermediate transfer device, since the image production is a continuous process. Since both the four-pass design and in-line design of the four-color printing apparatus are known in the art, further details regarding the configuration, construction, and operation of each need not be discussed.
Moving to FIG. 1, a schematic side elevation diagram of a typical prior art four-color laser electrophotographic imaging apparatus (xe2x80x9cprinterxe2x80x9d) 10 is depicted. The printer 10 comprises an intermediate transfer device 24. As is seen, the intermediate transfer device 24 can be configured as a movable, endless belt which is supported by a set of substantially parallel rollers 26. As can also be seen, the intermediate transfer device 24 can move, or revolve, in the direction xe2x80x9cA.xe2x80x9d
The prior art printer 10 also comprises an image-producing portion 12 which can sit astride the intermediate transfer device 24 as shown. The printer 10 can also include an indexing device 14 which can be configured to assist in moving and positioning sheets of finished product medium xe2x80x9cMxe2x80x9d as the sheets feed through the printer 10 in the direction xe2x80x9cB.xe2x80x9d The printer 10 can further comprise a transfer module 16 and a cleaning station 18, both of which are more fully described below. A sensor 20 can be positioned as shown and is also described in greater detail below. A viewpoint 50 is shown, from which the intermediate transfer device 24 can be observed over an elapsed time period, and which will be discussed in further detail below.
The prior art printer 10 can be configured as a four-pass design, or as an in-line design, among other designs. Therefore, the image-producing portion 12 can alternatively comprise a single laser/photoconductor as in the case of a four-pass configuration, or a plurality of laser/photoconductors as in the case of an in-line configuration. That is, for purposes herein, the image-producing portion 12 is meant to include any device which is configured to develop an image from toner and deposit the image onto the intermediate transfer device 24.
Although not specifically shown herein, it is understood that other configurations of prior art printers exist, such as those which deposit an image directly onto a finished product medium rather than onto an intermediate transfer device. However, the principles of operation of such other prior art printers are similar to those of the prior art printers depicted and discussed herein.
As is seen in FIG. 1, the image-producing portion 12 is shown in normal print mode and has produced a first completed output image P1 on the intermediate transfer device 24. The first output image P1 is depicted as being transferred onto a sheet of finished product medium xe2x80x9cM,xe2x80x9d which has moved in the direction xe2x80x9cBxe2x80x9d past the indexing device 14. The transfer module 16 causes the first output image P1 to become substantially transferred from the intermediate transfer device 24 to the sheet of finished product medium xe2x80x9cMxe2x80x9d as the sheet continues to move in the direction xe2x80x9cB.xe2x80x9d
The sheet of finished product medium xe2x80x9cM,xe2x80x9d on which the first output image P1 is transferred, can then continue on to a fuser (not shown) which can fuse the first output image to the medium. Meanwhile, the image-producing portion 12 has begun to produce a second output image P2 on the intermediate transfer device 24. Another sheet of finished produce medium xe2x80x9cMxe2x80x9d moves in direction xe2x80x9cBxe2x80x9d and into position to accept the second output image P2.
As is further seen in FIG. 1, a space xe2x80x9cSxe2x80x9d can be maintained between successive sheets of finished product medium xe2x80x9cM.xe2x80x9d The space xe2x80x9cSxe2x80x9d is typical for all printer apparatus and can aid in the proper movement and positioning of the sheets relative to the output images P1, P2 which, in turn, can provide for proper final alignment of the output images P1, P2 on the sheets. Apparatus and methods of positioning output images P1, P2 on finished product medium xe2x80x9cMxe2x80x9d in conjunction with printer apparatus are known in the art and need not be discussed herein.
FIG. 2 shows an additional view of the schematic side elevation diagram depicted in FIG. 1. In FIG. 2 it is seen that the printer 10 has begun to perform a calibration cycle. The image-producing portion 12 has started to produce a calibration image comprising a series of calibration patches 28 on the intermediate transfer device 24. A sheet of finished product medium xe2x80x9cMxe2x80x9d is held in position at the indexing device 14 and is not allowed to proceed to the transfer module 16 because the normal print mode is suspended during the calibration cycle. As the calibration patches 28 move past the sensor 20, the sensor can detect various characteristics of the calibration patches such as color density and the like.
During the calibration cycle, the sheet of finished product medium xe2x80x9cMxe2x80x9d can remain in position at the indexing device as shown without moving past the transfer module 16. Because no finished product medium xe2x80x9cMxe2x80x9d is available at the transfer module 16, the calibration patches remain on the intermediate transfer medium 24 as they pass the transfer module. Continuing past the transfer module 16, the calibration patches 28 reach the cleaning station 18 where the calibration patches are removed from the intermediate transfer device 24.
The removed calibration patches 28 are typically collected and deposited in a waste hopper 19. During the calibration cycle, only calibration patches 28 are produced by the image-producing portion 12. That is, during the calibration cycle of the prior art printer 10 no output images P1, P2 are produced. Typical elapsed times of prior art calibration cycles can be from one (1) to four (4) minutes or longer.
Turning now to FIG. 3, an time-lapse diagram is shown of the intermediate transfer device 24 as observed from the viewpoint 50 (shown in FIGS. 1 and 2) over a given time interval from time reference T1 to time reference T4. That is, FIG. 3 depicts what would be seen by an observer viewing the intermediate transfer device 24 from the viewpoint 50 during a time interval that starts at a time reference T1 and ends at a time reference T4.
It is noted that the diagram depicted in FIG. 3 reveals the output images P1, P2, P3, P4 that would normally be hidden from view by the finished product medium xe2x80x9cM.xe2x80x9d However, as is evident, the output images P1, P2, P3, P4 are included for clarity, and the perimeters thereof are delineated by dashed lines which indicate the xe2x80x9chiddenxe2x80x9d status of the output images.
At a first time reference T1 an observation of the intermediate transfer device 24 begins. Time elapses in the direction xe2x80x9cT.xe2x80x9d The first output image P1 is produced and transferred to a sheet of finished product medium xe2x80x9cMxe2x80x9d shortly before the first time reference T1. Shortly after the first time reference T1, the first output image P1 passes before the viewpoint 50 (shown in FIGS. 1 and 2). Shortly after the first output image P1 is observed, the second output image P2 is observed, having also been transferred onto a sheet of finished product medium xe2x80x9cM.xe2x80x9d
A study of FIG. 3 will reveal that the sheets of finished product medium xe2x80x9cMxe2x80x9d onto which the first and second output images P1, P2 are transferred, are separated by an inter-page gap xe2x80x9cG.xe2x80x9d By xe2x80x9cinter-page gapxe2x80x9d I mean an area which is on a surface, and which is situated either between two consecutive output images that concurrently reside on the intermediate transfer device, or between the edges of a single output image that wholly resides on the intermediate transfer device, and within which area no toner is to be transferred to any finished product medium.
By xe2x80x9csurface,xe2x80x9d I mean any surface which is configured to support an output image or a calibration patch. By xe2x80x9coutput image,xe2x80x9d I mean any image, group of images, or portion of an image, including a color plane, or color planes, comprising toner and intended to be transferred, to the exclusion of any other image, to a single piece of finished product medium. The inter-page gap xe2x80x9cGxe2x80x9d corresponds to the space xe2x80x9cSxe2x80x9d (shown in FIG. 1) which is maintained between each of two consecutive sheets of finished product medium xe2x80x9cM.xe2x80x9d
At a second time reference T2 a plurality of calibration patches 28 is observed beginning to pass the viewpoint 50 (shown in FIGS. 1 and 2) which passage indicates the beginning of a calibration cycle. At a third given time reference T3 the plurality of calibration patches 28 is observed to end, which indicates the end of the calibration cycle. A period of up to four minutes between the time references T2 and T3 can be typical for the elapsed time of a calibration cycle of prior art printing apparatus. The calibration patches 28 can comprise individual calibration patches of each of the toners available. That is, during a complete prior art calibration cycle, all four toners of a four-color printer 10 are included in the calibration patches 28.
Also, as seen, all calibration patches 28 that are produced during a prior art calibration cycle are produced between the same two consecutive output images, which in this case are P2, P3. As time elapses past time reference T3, the third output image P3 is observed to pass the viewpoint 50 (shown in FIGS. 1 and 2), followed by the passing of a fourth output image P4. The third and fourth output images P3, P4 are separated by an inter-page gap xe2x80x9cGxe2x80x9d which also corresponds to a space xe2x80x9cSxe2x80x9d which separates the sheets of product medium xe2x80x9cMxe2x80x9d onto which the third and fourth output images P3, P4 are transferred.
Turning now to FIG. 4, a flow diagram is depicted which shows the basic steps that can be performed in a prior art calibration cycle. The first step S1 is to initiate the calibration cycle. The second step S2 can be to stop the in-feed of the finished product medium xe2x80x9cMxe2x80x9d (shown in FIGS. 1-3) so as to suspend the normal print mode. The next step S3 is to begin producing the calibration patches 28 (shown in FIGS. 2 and 3), followed by the fourth step S4 which is to begin detecting and measuring the characteristics of the calibration patches. An example of a characteristic that would be measured in the fourth step S4 is the color density. The next step S5 is to finish producing the calibration patches 28 (shown in FIGS. 2 and 3), which can be followed by the sixth step S6 of finishing the detection and measurement of the characteristics of the calibration patches.
When the characteristics of the calibration patches 28 (shown in FIGS. 2 and 3) are measured, the next step S7 can be to compare the measured characteristics of the calibration patches to the characteristics of the reference calibration image (not shown). The eighth step S8 can be to query whether adjustments should be made to the printer 10 (shown in FIGS. 1 and 2) based on the comparison performed in the previous step S7. If the determination is made to make such printer adjustments, the method can proceed to the ninth step S9 which is to make the adjustments.
Once the adjustments are made, the method can proceed to the tenth step S10 which is to allow in-feed of the finished product medium xe2x80x9cMxe2x80x9d (shown in FIGS. 1-3). If the query of the eighth step S8 results in a determination not to make printer adjustments, the method can skip the ninth step S9 and proceed directly to the tenth step S10. After the tenth step S10, the calibration cycle can be ended at the final step S11. It is evident from FIG. 4 that the prior art calibration method excludes the production of normal printer output. That is, during the prior art calibration cycle, normal output images P1, P2, P3, P4 (shown in FIG. 3) are not produced.
As previously mentioned, calibration cycles of prior art printers can typically have elapsed times of between one (1) and four (4) minutes, or longer, depending on the configuration of the printer among other factors. During the calibration cycle, the normal print mode of the printer is suspended, or interrupted, until the calibration cycle is either completed or aborted. That is, during the calibration cycle, the printer is unavailable for producing normal output images. In high-demand or time-sensitive printer applications, such suspension of the normal print mode can be very undesirable for obvious reasons. It is therefore desirable to find a way to reduce or eliminate the unavailability of printers caused by the production of calibration images.
The invention includes methods and apparatus for utilizing an inter-page gap to produce calibration patches therein, thus substantially reducing or eliminating the interruption of the normal print mode of color laser printers caused by the calibration cycle.
In accordance with a first embodiment of the present invention, an imaging apparatus in accordance with the present invention is generally configured as an in-line type imaging apparatus and includes an intermediate transfer device. The apparatus comprises a computer memory which contains an algorithm for defining an inter-page gap on the intermediate transfer device. The algorithm can cause the apparatus to produce at least one calibration patch within the inter-page gap. The algorithm can also be configured to determine the number of calibration patches to produce within a given inter-page gap and to dictate the size of the given inter-page gap based on the number of calibration patches to be produced therein.
In accordance with a second embodiment of the present invention, an imaging apparatus is generally configured as a four-pass type and comprises an intermediate transfer device. The apparatus comprises a computer memory that includes an algorithm which is configured to determine the size of an inter-page gap that is located between the ends-of an output image which resides wholly on the intermediate transfer device. The algorithm can also cause at least one calibration patch to be produced within the inter-page gap. The number of calibration patches to be produced within the gap can be determined by the algorithm and based on the size of the inter-page gap.
In accordance with a third embodiment of the present invention, an imaging apparatus is generally configured without an intermediate transfer device. The apparatus comprises a computer memory that includes an algorithm which is configured to determine the size of an inter-page gap that is located on a photoconductor. The algorithm can also cause at least one calibration patch to be produced within the inter-page gap. The number of calibration patches to be produced within the gap can be determined by the algorithm and based on the size of the inter-page gap.
In accordance with a fourth embodiment of the present invention, a method of calibrating a color laser printer is disclosed. The method includes producing at least one calibration patch within at least one inter-page gap. The method can also include defining a plurality of successive inter-page gaps and producing at least one calibration patch within each of the plurality of gaps. Furthermore, the method can include dictating the size of each of the plurality of successive inter-page gaps and can also include dictating the size of each of the plurality of successive inter-page gaps based on the number of calibration patches to be produced within the gap.
In accordance with a fifth embodiment of the present invention, another method of calibrating a color laser printer is disclosed. The method includes determining the size of an inter-page gap based on the length of the surface on which the inter-page gap is defined. The size of the inter-page gap can also be determined based on the length of the output image to be produced. The method can also include producing at least one calibration patch within the inter-page gap and during each of a number of revolutions of the surface, wherein a different calibration patch is produced within, and selectively removed from, the inter-page gap during each revolution.